1. Field of the Invention
The present invention relates to illumination devices to be used for photography with video cameras or film cameras and, more particularly, to reduction of the size of such devices and increase of the efficiency with which to condense the illuminating light beam.
2. Description of Related Art
The illumination device for use with cameras or like photographic apparatuses is conventionally constructed with a light source (flash discharge tube), a reflector behind the light source and an optical member such as Fresnel lens.
In such an illumination device, as the light source radiates rays of light to all directions, a means must be provided for condensing the rays to a required angular field. So, many previous proposals have been made for increasing the efficiency at which to use flash light.
In particular, recently, the Fresnel lens that has so far been used in front of the light source is being replaced by what utilizes total reflection such as prism-light guide or like optical member. With this optical member, the beam can be condensed with a higher efficiency. In addition, the size of the illumination device can be reduced to a compact form.
In Japanese Laid-Open Patent Application No. Hei 4-138438, the optical member has a lens of positive refractive power for condensing that part of the light issuing from the light source which goes forward and a total-reflection surface from which another part of the light which radiates sideways is totally reflected to the front. With this arrangement, an illumination optical system proposed here is made to have a common exit surface.
In other words, the light from the light source enters the optical member at a surface thereof where its path splits in two. The two beams are brought to emerge from a common exit surface. For this purpose, use is made of prisms of reduced size with high efficiency of condensing the light beam.
Another Japanese Laid-Open Patent Application No. Hei 8-262537 proposes an illumination optical system as derived from that described above by limiting the values of the parameters and also by forming a vertical Fresnel lens in the exit surface of the optical member.
Yet another Japanese Laid-Open Patent Application No. Hei 8-234277 proposes an illumination optical system comprising light emitting means and optical means. To illuminate the subject, the light from the emitting means is conducted through the optical means of long length, by which it is bent to the subject. In such a manner, the efficiency of utilizing the light is improved.
In another Japanese Laid-Open Patent Application No. Hei 10-115852, the light beam from the light source is dispersed by an optical member to a certain range of angles of illumination. The optical member has an entrance surface confronting the light source, a reflection surface from which the entering light is partially reflected and/or part of it is totally reflected and an exit surface confronting the entrance surface and having a larger area than that of the entrance surface.
In recent years, the cameras or like photographic apparatuses are becoming smaller in size and lighter in weight themselves on one hand. Their taking lenses, on the other hand, tend to increase in the zoom ratio, for the purpose of letting them have a high value added. Because of this, if no auxiliary light source is used in shooting the camera, the exposure time has to increase when the lighting situations are equivalent to those heretofore encountered. As the image shake is liable to occur, the photographer will often fail to get the pictures he or she intends.
To save such situations, the camera is usually provided with an illumination device (flash device) as incorporated therein. For the reason described above, the frequency of using the flash device has been far more increasing than was heretofore possible, and the required amount of flash light for one shot has tended to increase.
In other words, suppose, in the lighting situations which are usually encountered, for two shots to take, the flash device is fired once, then the electric energy used up in this illumination accounts for about 80% of the total sum of energy the camera has consumed in carrying out all operations.
The increase of the frequency of firing the illumination device as described before accelerates increase of the ratio of the electric energy the illumination device consumes to that required to operate the whole apparatus. As the taking lens becomes darker, a larger amount of flash light is necessary to photograph a subject at an equivalent distance under an equivalent illumination.
On the other hand, along with the tendency to minimize the size of the camera, there are demands for reducing the size of the illumination device to a compact form. Accordingly, the problems of the illumination device of this character have become severer than were not so far found.
Under such a background, Japanese Laid-Open Patent Application No. Hei 4-138438 has made a proposal for using an optical member as arranged in front of a flash discharge tube. This optical member is provided with two surfaces, upper and lower, by which the rays dispersing sideways from the flashtube are brought to the entrance surface, and then totally reflected, thus being condensed to a certain direction. Besides this, at the front, there is a surface having a positive refracting power for condensing the light beam.
After the dispersing beams are condensed by the respective surfaces, they emerge from a common exit surface. In this proposal, concerning the beam condensation in the longitudinal direction of the flashtube, nothing particular is suggested.
To improve this proposal, Japanese Laid-Open Patent Application No. Hei 8-262537 employs a Fresnel lens in the exit surface, thus showing a method of condensing the light beam in the longitudinal direction of the flash discharge tube. In the illumination device of the character described, the optical system has to be reduced in size extremely. For this purpose, the Fresnel lens is merely employed. However, it is difficult to produce a good effect of condensing the light beam.
In more detail, the Fresnel lens is positioned at the front extremely closer to the light source than in the conventional method. Further, the space between them is filled not with the air as in the conventional example but with plastic or like optical material. For this reason, the optical path cannot be taken substantially long enough. So, it becomes impossible for the Fresnel lens to assure production of the necessary effect of condensing the light beam. Moreover, that part of the light which does not engage in the condensed light beam at the edges of the Fresnel lens increases, thus going to the outside of the intended angular field.
The present invention is concerned with the illumination optical system in the photographic apparatus. The entirety of the illumination optical system is reduced to a compact size, while still keeping the optical characteristic as it stands. It is, therefore, an object of the invention to provide an illumination device which can increase the illumination, as the light beam is more effectively cast into the target area, and to provide a photographic apparatus using the illumination device.
To condense a light beam, there have been no methods except to increase the size. Another object of the invention is, therefore, to provide an illumination device which can remarkably improve the characteristics of condensing the light beam in the longitudinal direction of the light source (or along the axis of the flash discharge).
Still another object of the invention is to provide an illumination device which is reduced in size, is thinned in bulk and is made lighter in weight, while nonetheless making it possible to utilize the energy from the light source with a higher efficiency and, therefore, which is suited to still cameras, or video cameras, and to provide a photographic apparatus using the illumination device.
To attain the above objects, in accordance with an aspect of the invention, there is provided an illumination device, comprising light source means, and an optical member having an entrance surface which a light beam from the light source means is made to enter and an exit surface from which the light beam having entered the entrance surface is made to exit, wherein a plurality of prism portions are arranged at the exit surface of the optical member, and wherein a part of the plurality of prism portions totally reflect a part of the light beam having entered the entrance surface.
Further, in the illumination device, the part of the plurality of prism portions are prism portions corresponding to a central region of light emission of the light source means.
In particular, an apical angle xcex8 of each of the plurality of prism portions lies within the following range:
2xc3x97{sinxe2x88x921(1/n)}xe2x89xa6xcex8xe2x89xa690xc2x0+sinxe2x88x921(1/n)
where n is a refractive index of material of the plurality of prism portions.
Further, a pitch interval of the plurality of prism portions is not less than 0.2 mm and not more than 2 mm.
Further, the optical member further has an additional entrance surface and a total-reflection surface which totally reflects a light beam having entered the additional entrance surface to lead the light beam to the plurality of prism portions.
Further, the plurality of prism portions have different apical angles for every different region thereof, and the prism apical angle in a central region is larger than the prism apical angle in a marginal region.
Further, the light source means has a tube-like shape, and the plurality of prism portions are formed such that a ridgeline forming each apical angle of the plurality of prism portions is approximately perpendicular to a longitudinal direction of the tube-like shape.
Further, the light source means has a tube-like shape, and the plurality of prism portions are formed such that a ridgeline forming each apical angle of the plurality of prism portions is inclined to a predetermined angle which is within a range of 45xc2x0 to 90xc2x0 with respect to a longitudinal direction of the tube-like shape.
Further, the light source means has a tube-like shape, and a reflector of approximately cylindrical shape is disposed immediately behind the light source means.
Further, the reflector has a cylindrical surface and a flat surface, and the flat surface is in contact with the optical member.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.